1. Field of the Invention
The present invention generally relates to sockets for mounting packages on printed-circuit boards, and more particularly to a socket for mounting a high-density package having hundreds of I/O pins on a printed-circuit board.
2. Description of the Related Art
Sockets have been conventionally used for mounting a conventional package having a relatively small number of I/O pins on a printed-circuit board. The socket comprises a socket body formed of synthetic resin and a plurality of contacts. The contacts are integrated in the socket body in an arrangement corresponding to the I/O pins of the package. The socket is mounted on the printed-circuit board by soldering lead parts of the contacts to pad patterns formed on the printed-circuit board. The package is mounted on the socket by inserting the I/O pins into I/O pin contact parts of the contacts.
The conventional socket has greater flexural rigidity than that of the printed-circuit boards. In this specification, the flexural rigidity of a socket or a printed-circuit board corresponds to a force which is required to cause a specific amount of bending of the socket or the printed-circuit board. That is, if a larger force is required to cause the specific amount of bending, the flexural rigidity is greater. On the contrary, if a smaller force is required to cause the specific amount of bending, the flexural rigidity is smaller.
Recently, attempts have been made to mount high-density packages (e.g., multi-chips) having hundreds of I/O pins by using sockets. The high-density packages are generally of a larger size than the conventional packages while the pitch of I/O pins of the high-density packages is as small as the conventional packages. For example, the longitudinal length of the high-density packages is typically as much as 100 mm and the pitch of the I/O pins is only about 1 mm. Accordingly, sockets for mounting the high-density packages are of a larger size with the longitudinal length being about 100 mm for example, while the pitch of lead parts of contacts is as small as 1 mm.
Therefore, if sockets for mounting the high-density packages are designed in accordance with the socket technology for the conventional packages, bending of the sockets may have a considerable effect on the reliability of soldering connections between the lead parts of the contacts and the pad patterns on the printed-circuit board. In this case, the sockets may not be mounted on the printed-circuit board with sufficient strength.
FIG. 1 is a diagram showing the above problem of the conventional sockets. A socket 10 is mounted on a printed-circuit board 11 with lead parts 12 being soldered to pad patterns 13 formed on the printed-circuit board 11. The solder is indicated by a reference numeral 14. A high-density package 15 is mounted on the socket 10. As shown in FIG. 1, the socket 10 is bent with its center part being downward. Thus, the lead parts 12 which are disposed near the edge of the socket 10 have been displaced from the center O.sub.1 of the pad patterns 13 so that the lead parts 12 face the edge part of the pad patterns 13, as shown in an enlarged and exaggerated manner in FIG. 1. In this case, the lead parts are soldered only on a portion of the circumference thereof and thus the amount of the solder 14 which connects the lead parts 12 to the pad patterns 13 is reduced. Therefore, some of the lead parts 12 can not be soldered to the pad patterns 13 with sufficient strength.
As the size of a socket becomes larger, the socket tends to bend more easily. It is difficult to suppress the bending of the socket even if the thickness of the socket is reduced by modifying the shapes of molds which are used in molding the socket.
When an electronic apparatus which contains the circuit board 11 with the high-density package 15 mounted thereon is subjected to a shock, cracks may occur at portions where the lead parts 12 are not soldered to the pad patterns 13 with sufficient strength. In this case, electric conduction between the lead parts 12 and the pad patterns 13 may be broken, and thus the operation of the electronic apparatus may be stopped.
In addition, the printed-circuit board 11 tends to bend due to heat generated by the operation of the electronic apparatus which contains the printed-circuit board 11. Conventional sockets in general have greater flexural rigidity than that of printed-circuit boards. For this reason, the socket 10 does not bend to follow the bending of the printed-circuit board 11. Thus, when the printed-circuit board 11 has bent in a direction A, a considerably large force F.sub.1 is generated which biases the lead parts 12 out of the pad patterns 13, as shown in FIG. 1. The force F.sub.1 may break the soldering connections between the lead parts 12 and the pad patterns 13.